The technique related to a conventional damper for a speaker and a speaker using the damper will be described with reference to FIG. 4, FIGS. 5A and 5B, and FIGS. 6A and 6B.
FIG. 4 is a cross section of a conventional speaker. In FIG. 4, a magnetic circuit is constructed by plates 43 and 44 and magnet 42. When current according to a sound signal flows in voice coil 41 disposed in a magnetic gap, a force according to the current flowing in voice coil 41 and density of magnetic flux interlinked with voice coil 41 acts on voice coil 41. Since diaphragm 45 is coupled to voice coil 41, it moves integrally with voice coil 41. Diaphragm 45 and voice coil 41 are supported by edge 47 and damper 46, respectively. Edge 47 and damper 46 change their shapes and are movable in the vertical direction, thereby moving diaphragm 45 in the vertical direction and generating sound pressure.
In a general speaker, edge 47 has a large-curve shape which allows large amplitude. The stiffness of edge 47 itself is designed to be low. Since damper 46 supporting voice coil 41 needs a high center holding force so as not to be in contact with the magnetic circuit when voice coil 41 vibrates, damper 46 has a shape including a plurality of small curves. Usually, damper 46 is set to have stiffness higher than that of edge 47, and the stiffness of damper 46 is dominant in the whole speaker. One of the big factors exerting influence on distortion as a serious drawback of a speaker is nonlinearity caused by fluctuations in the stiffness. From the viewpoint of the distortion performance, it is important that stiffness is constant irrespective of fluctuations in diaphragm 45. Since the stiffness of damper 46 is dominant in that, in the entire speaker, the linearity in the vertical direction of damper 46 conspicuously exerts an influence on distortion of the speaker.
FIG. 5A is a plan view of damper 46 of the conventional speaker. FIG. 5B is a cross section taken along line C-C in FIG. 5A. As shown in FIG. 5B, for damper 46, wavy woven or nonwoven cloth in which a plurality of curved parts are formed in cross section and which is impregnated in a thermosetting resin is used.
FIG. 6A is a plan view of another conventional damper 46A. FIG. 6B is a cross section taken along line D-D of FIG. 6A.
As shown in FIG. 6B, damper 46A has a rise part from a face bonded to a frame and has a plurality of curved parts. Since there is the rise part, a large dimension of contact to the frame, of damper 46A can be realized. Since the distance of the damper from the part bonded to the frame to a part bonded to the voice coil can be made long, the amplitude in the vertical direction of the damper can be made large.
Known related prior art includes, for example, Unexamined Japanese Patent Publication No. H62-193399 and Tamon Saeki, “Encyclopedia of Speakers and Enclosures”, Seibundo Shinkosha Inc., May 28, 1999, p. 62.
However, the techniques have the following drawbacks. To increase the vertical amplitude of the damper itself, the radius of the curved part has to be increased. When the radius of the curved part is increased, the stiffness is increased by the shape of the curve itself. In a damper having a small diameter, it is difficult to realize increase in the radius of the curved part. Curves may be partly formed large. In this case, symmetry in the vertical direction deteriorates and stress is concentrated on the enlarged curves. There is consequently the possibility that the damper is destroyed from fatigue caused by vertical vibrations.
There is also prior art disclosing a method of using, as a damper material, a supporting member obtained by stacking a rubber layer as an elastic member on woven or nonwoven fabric in order to solve the problems. However, the mass of the damper itself increases and, as a result, the mass of the vibration system becomes large. There is a problem that the sound pressure decreases.